In recent years, soft silicone hydrogel contact lenses become more and more popular because of their high oxygen permeability and comfort. Most commercially available silicone hydrogel contact lenses are produced according to a conventional cast molding technique involving use of disposable plastic molds and a mixture of monomers in the presence or absence of macromers. However, disposable plastic molds inherently have unavoidable dimensional variations, because, during injection-molding of plastic molds, fluctuations in the dimensions of molds can occur as a result of fluctuations in the production process (temperatures, pressures, material properties), and also because the resultant molds may undergo non-uniformly shrinking after the injection molding. These dimensional changes in the mold may lead to fluctuations in the parameters of contact lenses to be produced (peak refractive index, diameter, basic curve, central thickness etc.) and to a low fidelity in duplicating complex lens design.
Such disadvantages encountered in a conventional cast-molding technique can be overcome by using the so-called Lightstream Technology™ (CIBA Vision) based on free-radical chain-growth polymerization, as illustrated in U.S. Pat. Nos. 6,800,225 and 8,088,313, which are incorporated by reference in their entireties. The Lightstream Technology™ involves (1) a lens-forming composition which is preferably a solution of one or more substantially purified prepolymer with ethylenically unsaturated groups and which generally is substantially free of monomers and crosslinking agents with a small molecular weight, (2) reusable molds produced in high precision, and (3) curing under a spatial limitation of actinic radiation (e.g., UV). Lenses produced according to the Lightstream Technology™ can have high consistency and high fidelity to the original lens design, because of use of reusable, high precision molds. In addition, contact lenses with high quality can be produced at relatively lower cost due to the short curing time, a high production yield, and elimination of lens extraction with an organic solvent.
U.S. Pat. Nos. 8,003,710, 8,071,703 and 8,283,429 (herein incorporated by reference in their entireties) disclose an improved Lightstream Technology™ based on thiol-ene step-growth polymerization. It is believed that polymeric materials obtained based on step-growth polymerization possesses a network constructed solely by a clean one-on-one chain extension reaction and having relatively large molecular mass between crosslinks. With high molecular mass between crosslinks, the polymeric material can have higher mechanical strength, such as, e.g., tear resistance.
In order to fully utilize the Lightstream Technology™ to make silicone hydrogel contact lenses, there is still a need for new actinically-crosslinkable prepolymers suitable for making silicone hydrogel contact lenses according to the Lightstream Technology™.